Why Do Worms Come Out When It Rains? Theories Explained

Do worms come out to avoid drowning?

The idea that worms flee flooded burrows is the most popular explanation, but it does not fit the evidence well. An earthworm has no lungs. Oxygen passes straight through its thin, moist skin into the blood, and that works in water as well as in air, as long as the water contains enough dissolved oxygen. The common earthworm (Lumbricus terrestris) has been kept alive underwater in such conditions for days and even weeks.

Worms also come up in light rain that does not flood the soil, and in some places they surface on damp nights when it is not raining at all. A drowning reflex would not explain this. Earthworm biologists now consider several other explanations more likely, and the real answer may be a mix of them.

Theory 1: rain makes it safe to travel

For an animal that breathes through its skin, dry air is dangerous. A worm caught on dry ground in sunshine loses water quickly and can die. Rain changes this. When the ground surface is soaked, a worm can crawl over it for hours without drying out.

Travel over the surface is much faster than tunneling through soil. Common earthworms have been recorded moving at around 20 m (65 ft) per hour on moist, flat ground, far quicker than they can dig. Wet nights therefore give worms a chance to move to new feeding areas, escape crowded patches and find mates that are not close relatives. On this view, worms on the pavement after rain are not fleeing but dispersing, and some of them get lost on hard surfaces.

Theory 2: raindrops feel like digging moles

Earthworms are extremely sensitive to vibrations in the soil. In 2008 the biologist Kenneth Catania studied why large earthworms in a Florida forest rush to the surface when local bait collectors make the ground vibrate. He found that eastern moles live in the same area and that the worms flee from digging moles by racing out of their burrows. The vibrations made by people closely resembled those of moles.

Some birds and other animals seem to exploit the same reaction: herring gulls patter their feet on grass, and wood turtles stamp on the ground, apparently to bring worms up. A number of researchers have suggested that the drumming of heavy rain might trigger a similar escape response. Catania’s study, however, found no support for the idea that worms mistake vibrations for rain and surface to avoid drowning, and the raindrop idea has not been firmly tested.

Theory 3: soggy soil runs short of oxygen

Water holds far less oxygen than air. When rain fills the small air spaces in the soil, oxygen can fall, especially in warm weather when soil microbes and roots use it up quickly. Carbon dioxide can build up at the same time.

A worm sitting in a burrow full of stagnant, oxygen-poor water may do better by moving up to the surface, where oxygen is plentiful. This could explain why worms often appear after long, heavy rain on heavy clay soils. It may be a factor in some conditions, but it does not explain worms surfacing during light showers.

Feeding and mating on damp nights

Many of the worms seen after rain are simply doing what they always do, only in larger numbers. The common earthworm, often called the nightcrawler, feeds by stretching out of its burrow at night to pull fallen leaves underground, and it mates on the surface with a neighbor while both keep their tails in their burrows.

Both activities are safest when the ground is wet and the air is cool and still. Rain makes these conditions widespread, so worms that would normally stay hidden in dry spells all come up at once. Because people notice them on paths in the morning, it looks as if rain has “driven” them out.

Why do worms die on sidewalks after rain?

Worms that wander onto pavement, roads and patios often cannot find their way back to soil. As the surface dries, they lose water through their skin. Earthworms are also sensitive to sunlight, and strong light, especially ultraviolet, can damage their skin. Many of the dried-out worms seen on sidewalks are the result of being stranded when the sun comes out.

Birds take advantage too. American robins, European blackbirds, thrushes and gulls feed heavily on lawns after rain, when worms are near or on the surface; how robins find worms explains their technique. Lapwings and some other birds even tremble their feet on wet ground while feeding, which may coax worms upward.

So why do worms come out in the rain?

No single experiment has settled the question. The best-supported answer is that rain removes the main danger of the surface, drying out, so worms use wet periods to travel, feed and mate. Vibrations from rain and low oxygen in waterlogged soil may add to the effect in some places.

What scientists agree on is that drowning is not the main reason. An animal that breathes through damp skin is well suited to wet conditions, and rainy nights are among the best times of the year for an earthworm to be above ground. How that skin breathing works is described in how many hearts an earthworm has.